The invention has to do with a procedure for filtration of fluids, particularly of heterodisperse suspensions like beer, using MF modules in which filtration cycles alternate with cleansing cycles, and during filtration the trans-membrane pressure is measured constantly.
Although the filtration process is applicable to any fluid, treatment of worts, green beer or aged beer is of primary significance in the invention.
Filtration of beer in the brewery represents an important procedural step to produce a product in accord with user's expectations. For this at present, diatomite and layer filtration are used. These, however, have a disadvantage in that filtration aids requiring disposal are produced to a greater extent. Essentially this applies to used diatomite, the consumption of which comes to about 150,000 tons per annum worldwide. The diatomite sludge to be disposed of has a dry substance content of only about 20 to 25%, so that the quantity of diatomite sludge to be disposed of totals at least four times the above tonnage.
Disposal of diatomite sludge causes major environmental problems. Along with the instability in dumps caused by diatomite sludge, percolating water of diatomite sludge in particular that is released represents a great environmental burden owing to its high BSB*and CSB*values. FNT *BSB and CSB are measures of biochemical oxygen consumption, and chemical oxygen consumption, respectively.
Quite some time ago efforts were made to find alternatives to the traditional treatment methods. Cross flow technology has already been investigated as an alternative, and achieved acceptance particularly in the area of wines. Until now this technology could not be used successfully in breweries, primarily owing to low specific surface performance and analytic changes of the filtered beer during filtration lasting many hours.
In the field of wine we have a possibility to markedly increase the flux rate in cross flow filtration by heating the wine to be filtered, as is known from DE 34 23 594 A1. If the products are heated to considerably above 35.degree., performance with the cross flow process can be raised to double what it was. In the area of beer filtration this measure is not possible, since beer must be filtered at temperatures from -1 to +3.degree. C., so that low-temperature-unstable substances such as certain proteins do not go into solution, thus later causing the beer to become cloudy when the consumer sees it.
Because of beer's particular contents, filtration of beer is considerably more difficult than filtration of wines, for example. In addition, beer contains coarse disperse particles such as yeast, or possibly present beer pests, or colloidal components. Primarily we are dealing here with high-molecular-weight compounds of proteins with carbohydrates, tannin and hops resins. Designated as a third component group are molecular disperse components with a particle size &lt;0.001 .mu.m.
According to G. E. Walla (Cross Flow Microfiltration in Breweries, dissertation in the Department of Technical Microbiology and Brewery Technology, Munich Technical University, 1992, p. 7), the following requirements exist for beer filtration:
1. That the filtrate have sufficient physico-chemical stability.
2. No bad-tasting product degradation from the filtration process.
3. Biological stability of the filtrate.
4. Sparkle quality and clean filtrate; preservation of CO.sub.2 content.
A further requirement has to do with the chemical analysis appearance of the filtrate. This is allowed to be changed by the filtration process only to a trivial extent.
Walla, in the course of his investigations, determined that a trans-membrane pressure can be regarded as optimal (dissertation, p. 66) for beer filtration as regards the components of the filtrate. Filtrate analyses have shown that the original wort content peaks at a trans-membrane pressure of 1.5 bar, as compared to higher or lower trans-membrane pressures.
Proceeding from this knowledge, Walla attempted to raise the flux rates while maintaining a constant trans-membrane pressure of 1.5 bar.
The flux is affected by the cover layer being formed on the membrane. Therefore an attempt was made to eliminate this by periodic back-rinsing. However, this procedure has a disadvantage in that as operating duration increase, the flux rate (both at the start and after back-rinsing with filtrate) is lower than in the previous cycle. This can be explained by the fact that certain adsorption effects take place which lead to a membrane blockage to a limited degree.
In addition, Walla determined (dissertation, p. 83) that in spite of periodic back-rinsing, particularly over a number of hours of filtration using the cross flow process, the original wort, the apparent extract, the thickness and foam stability in the filtrates decline. On average the original wort of the filtrate declined by an average of 0.5 weight % without periodic back-rinsing. With periodic back-rinsing the change was still 0.2 weight %. Without periodic back-rinsing the foam value dropped by 21 foam-points; using periodic back-rinsing, the drop was still 6 foam-points.
Another option for achieving larger flux rates consists in increasing overflow velocities (see Walla, Dissertation, page 62ff). From the investigations it can be clearly gleaned, however, that even a high overflow rate over a long period results in no significantly higher flux rate. The average performance after about six hours of filtration, even at a rate of 6 m/sec, amounted to only 35 L/m.sup.2 h. The cause of this is that even when higher overflow velocities are employed, buildup of a cover layer on the membrane surface cannot be totally avoided.
Additionally, the cover layer that forms considerably affects the filtration result by formation of a so-called secondary membrane in such a way that the selectivity of the actual membrane becomes greater. This means that even if membranes with a large nominal pore size are used, a cover layer is formed that is far under the nominal pore size of the membrane, so that essential components are removed from the beer.
High flow velocities, however, are to be avoided in beer filtration for the following reasons. First, overflow pumping causes an enormous amount of energy to be transferred into the system, so that unless there is supplemental cooling, beer heats up very rapidly. Through mechanical loading of colloidal substances in beer, particularly glucans, the filtration capacity of beer becomes steadily worse owing to pumping transfer in the cross flow system through gelation of .beta.-glucan.
A procedure is known from DE 39 36 797 C2 for separating beer from a raw material flow transferred during a fermentation process. In this process, ceramic membranes are used, for, in contrast to polymeric ones, ceramic membranes can be sterilized by hot water. These ceramic membranes are rinsed at large time intervals using a chemical cleaning solution, and are back-rinsed using hot water until free of chemicals in numerous intermediate cleaning steps. The cleaning steps are only carried out if a decrease in filtrate flow has taken place as a result of increasing blocking of the ceramic membrane. Also in this process there is a danger that possible essential components of the fluid to be filtered are filtered out by the cover layer, thus impairing the analytical appearance of the filtrate.
From DE 39 14 956 A1 is known a process for accelerating material exchange of a continuous bioreactor. In this process, using pressure modulations, formation of a secondary layer on the filtration membrane is prevented. Pressure changes are controlled depending on the pressure difference measured over the membrane. These changes must be oriented to providing the systems produced with fluid and nutrients.
What is recommended in Weinwirtschaft-Technik, 1990, pp. 15-21, is that in every case where the residue temperature increases too much, or filtration performance declines too much, that cross flow filtration be interrupted and a cleaning cycle be undertaken.